Process for the production of gold powder in platelet form

ABSTRACT

Gold powder is produced in flake form by reducing aqueous hydrochloric acid solution of chloroauric III acid with an unsaturated alcohol in the presence of a protective colloid.

United States Patent Lutz et a1. July 1, 1975 PROCESS FOR THE PRODUCTION OF GOLD POWDER IN PLATELET FORM ReferencesCited [75] Inventors: Klaus Lutz, Bergen-Enkheim; UNITED STATES PATENTS Manfred Golla, Grossauheim, bQth 3,201,223 8/1965 Cuhra Et al. 75/1 18 f Germany 3,401,186 9/1968 Muller et a1. 3,717,453 2/1973 Daiga 75/O.5 A [73] Assignee: Demetron Gesellschaft fur 3,725,035 4/1973 Short et a1. 75/118 Elecktronik-Werkstoffe mbH, 3,725,047 4/ 1973 Schneider 75/118 Frankfurt, Germany I Primary Examiner-W. Stallard [22] Ffled' May 1974 Attorney, Agent, or Firm-Cushman, Darby & [21] Appl. No.: 475,280 Cushman [30] Foreign Application Priority Data [57] ABSTIFACT June 15 I973 German 2330413 Gold powder 15 produced m flake form by reducing y aqueous hydrochloric acid solution of chloroauric 111 [52] U S Cl 75/0 5 75/118 R acid with an unsaturated alcohol in the presence of a 51 lm. c1 C22b 11/06 Pmtectwe [58] Field of Search 75/05 A, l 18 9 Claims, No Drawings PROCESS FOR THE PRODUCTION OF GOLD POWDER IN PLATE-LET FORM The invention concerns a process for the-production of flake'(platelet) goldpowder by reduction of aqueous hydrochloric acid solutions of chloroauric (lll) acid (HAuCl with unsaturated alcohols, i.e., ethylenically or acetylenically unsaturated alcohols, in the presence of a protective colloid. *1

Gold powder of various "particle sizes and shapes is employed in many areas of the arts, as for example,"for the production of decorative articles, ele'ctrical conductors and resistanc'es,or for printed circuits;

In the previously knowh"proc'ss for the production of gold powders by the'reduction of gold salt-solutions with chemical reducing agents, the powder forms as irregular particles, frequently even as needles. 1n the industrial use of gold powder, especially for conductive pastes, however, it has beenshown that flake particles are better suited for such purposes than powders which consist of other particle formsj For the production of flake type gold powde rQc us'tornarily crystalline powder is started with and it is ground with steel balls 'in'a ball mill for a long time. Suitably, there are added grinding assisting agents as, for example, fatty acids or organic hydrocarbon, Short US. Pat. No. 3,539,114. Because of the ductility of the gold, the starting crystallites are shaped thereby to more or less large flakes. However, these processes have the disadvantage that they are very time-consuming and a product is produced that does not fulfill all of the requirements placed on a true flake form of gold powder.

Therefore, it was the problem of the present invention to find a process which permits the direct production in a chemical way of a flake form of gold powder directly from the solution of a gold salt.

It has now been found according to the invention that there can be obtained well formed, flake gold powder directly by reduction of gold solutions if one starts with an aqueous, hydrochloric acid solution of chloroauric (111) acid and reduces this with an ethylenically or acetylenically unsaturated alcohol in the presence of a protective colloid.

As the protective colloid there is preferably used gum arabic in an amount of 2 to 6% of the mixture. However, there can also be used other colloid constituents such as dextrin methyl cellulose, tannic acid, gum tragacanth, sodium algenate, potassium algenate, gelatin, kolophonium etc. The amount of protective colloid can also range from 2 to 15 of the weight of the mixture.

As the reducing agent there can be used any ethylenically or acetylenically unsaturated alcohol, e.g., a1- kenols such as allyl alcohol, methallyl alcohol, crotyl alcohol, oleyl alcohol, linolyl alcohol or alkinols such as propargyl alcohol or citronellol.

Expecially valuable are alcohols having a double bond (or triple bond) in the alpha position, especially allyl alcohol and/or propargyl alcohol. There can also be used as reducing agents enediols and other organic Compounds which have an OH group adjacent to a double bond.

Preferably the reduction to gold powder takes place at a pH of between and 4. The hydrochloric acid solution containing chloroauric (Ill) acid must contain at least of gold by weight. With solutions containing less than 5% gold, there is no separation of flake shaped powder. There is no critical upper limit on the amount of gold, e.g., itcan be present up. to a saturatedsolution. By variation of the reduction parameters such as temperature or pH value, the size of the flakes can be influenced i'n -the rage of, for example, 1 to 50 mp whereby the gold crystallizes in the form of triangular or hexagonal flakes. I

Generally, the reduction takes place by. slowly dropping the reducing agent into a solution mixture of chloroauric (Ill) acid and protective colloid. The amount of reducing agent added depends on the gold content of the'solution. Preferably the amountof reducing agent, however, is greater than that stoichio'nietrically needed for reaction with the gold in order that the'reduction is quantitative. Very finely crystalline gold flakes are obtained if the chloroauric (111) acid solution is treated with the reducing agent at low temperatures (e.g., room temperature such'as-1825C.) diluted' with water and the solution allowed to decolorizerThen the' mix ture.is heated to C. whereupon the gold powder precipitates. The temperature can this range from room temperature to the boiling point.

The flake shaped gold powder produced by the process of the invention is especiallysuited for decorative and thick film-conducting pastes because of its good screen printing properties and the high electrical conductivity of the baked conductor path.

Unless otherwise indicated, all parts and percentages are by weight.

The process of the invention will be further explained in the following examples.

EXAMPLE 1 3.3 kg of an HAuCl solution containing about 33% Au was diluted with soda lye to pH 0.5-1 and treated with 2 kg of aqueous 30% gum arabic solution. There was added with stirring at a maximum temperature of 30-35C. 1 liter of allyl alcohol in the course of 90 minutes. The reduction mixture was held at room temperature until the first appearance of gold flakes, then warmed to 6070C. and held there for about 15 minutes. The gold powder was washed by decantation with water and alcohol, filtered with suction and dried at 70C. There was obtained thereby a quantitative yield of crystalline flakes of particle size of 25 mp or less.

EXAMPLE 2 A mixture of 750 ml of allyl alcohol and 250 ml of propargyl alcohol was dropped into a mixture of 3.3 kg of 30% aqueous HAuCl, solution (pH 1-1.5) and 2 kg of aqueous 30% gum arabic solution with stirring at 70-80C. within 90 minutes. Further reaction was allowed to continue for 10 minutes at this temperature. There were obtained thereby crystalline flakes having a particle size of not over 15 mu.

EXAMPLE 3 To 1.6 kg of an aqueous HAuCl, solution (pH 1-1.5) and 1 kg of a 30% aqueous gum arabic solution there were dropped in a mixture of 500 ml of allyl alcohol and 500 ml of water at 5060C. during 4 hours. There were obtained crystalline flakes of particle size of 2 to 50 mu.

EXAMPLE 4 A mixture of 3.3 kg of aqueous HAuCl solution (pH 1 2 kg of 30% aqueous gum arabic solution and 1 liter of allyl alcohol were diluted with water in the ratio of 1:1 by volume. The mixture was held with stirring at a maximum temperature of 30C. until the solution had decolorized. Then it was warmed to 80C. and there was obtained a gold powder which consisted of flakes having a particle size of mu or less.

EXAMPLE 5 3.3 kg of an aqueous HAuCl solution containing about 33 Au was diluted with a aqueous solution of NaOH to pH 5 and treated with 1,2 kg of an aqueous 30 dextrine solution. There was dropwise added 1 kg ascorbic acid to this mixture. There was obtained thereby crystalline flakes of particle size of 10 60 um.

EXAMPLE 6 flakeform comprising reducing an aqueous hydrochloric acid solution of chloroauric (III) acid with an ethylenically or acetylenically unsaturated alcohol in the presence of a protective colloid. V

2. A process according to claim. 1 wherein the alcoho] is an alkenol. v

3. A process according to claim 1 wherein the alcohol is an alkineol. 1

4. A process according to claim 1 wherein the. alcohol is an alkeneol or alkineol and the hydroxyl group is attached to a carbon atom alpha to a carbon atom containing the ethylenic or acetylenic unsaturation.

5. A process according to claim 1 wherein the solution has a pH between 0 and 4. v I

6. A process according to claim 5 wherein the protective colloid is gum arabic.

7. A process according to claim 6 wherein the reduc' ing agent is allyl alcohol, propargyl alcohol or a mixture of allyl alcohol and propargyl alcohol.

8. A process according to claim 5 wherein the unsaturated alcohol is allyl alcohol. 3

9. A process according to claim 5 wherein the unsaturated alcohol is propargyl alcohol. 

1. A PROCESS FOR THE PRODUCTION OF GOLD POWDER IN FLAKE FORM COMPRISING REDUCING AN AQUEOUS HYDROCHLORIC ACID SOLUTION OF CHLOROAURIC (III) ACID WITH AN ETHYLENICALLY OR ACETYLENICALLY UNSATURATED ALCOHOL IN THE PRESENCE OF A PROTECTIVE COLLOID.
 2. A process according to claim 1 wherein the alcohol is an alkenol.
 3. A process according to claim 1 wherein the alcohol is an alkineol.
 4. A process according to claim 1 wherein the alcohol is an alkeneol or alkineol and the hydroxyl group is attached to a carbon atom alpha to a carbon atom containing the ethylenic or acetylenic unsaturation.
 5. A process according to claim 1 wherein the solution has a pH between 0 and
 4. 6. A process according to claim 5 wherein the protective colloid is gum arabic.
 7. A process according to claim 6 wherein the reducing agent is allyl alcohol, propargyl alcohol or a mixture of allyl alcohol and propargyl alcohol.
 8. A process according to claim 5 wherein the unsaturated alcohol is allyl alcohol.
 9. A process according to claim 5 wherein the unsaturated alcohol is propargyl alcohol. 